1. Field of the Invention
This invention relates to the indirect measurement of optical density, and to the use of the measurement to assay solutions of a material which imparts optical density at a given wavelength or band of wavelengths. The actual measurement that is made is of scattered energy intensity; the ratio of two of these intensities has been found to be a function of optical density or absorbance.
2. The Prior Art
So far as is known, the measurement of scattered energy intensity has not heretofore been used to determine optical density. Scattered energy intensity measurements have been made, in reducing the instant invention to practice, using a retrofitted "TDx", a registered trademark, instrument that is commercially available from Abbott Laboratories. The instrument, insofar as its use is relevant to the instant invention, comprises a light scattering accessory carousel which carries twenty cuvettes for samples to be investigated, automatic pipetting means for adding material to the cuvettes, and a sensor for measuring the intensity of light energy leaving each of the cuvettes in succession as the carousel is rotated from position to position. The original instrument also includes means to excite samples in the cuvettes with light of a particular wavelength, but this means plays no part in the practice of the instant invention; its purpose in the original instrument is to excite a sample, causing luminescence or fluorescence. The measured intensity of the energy leaving a cuvette as a consequence of the luminescence or fluorescence could be used in analytical work, for example to determine the concentration of the luminescing or fluorescing material in a sample or to measure the polarization of fluorescence.
Two different instruments were retrofitted for use in practicing the instant invention. In one case light emitting diodes were installed radially inwardly of the first six cuvettes in a carousel for the instrument, and were connected in parallel through a 90 ohm load resistor and a nickel-cadmium 9 volt battery. Each of the diodes was mounted to direct a beam of light through a sample in the adjacent cuvette; the axis of the beam of light was in the same plane as the axis of the sensor for the detection of scattered light intensity and at an angle of 37.5.degree. thereto. In the other case, a standard carousel for the instrument was modified so that an inner portion of the base remained stationary while the rest of the carousel (including the 20 cuvettes) rotated therearound during operation of the instrument. A single light emitting diode and a constant voltage regulator circuit were installed upon the stationary base. The circuit included a load resistor and a lithium battery. The light emitting diode was so positioned that the axis of the light beam therefrom was in the same plane as the axis of the sensor for the detection of scattered light intensity and at an angle of 37.5.degree. thereto. When the carousel was rotated, successive ones of the cuvettes were positioned for the light beam to be directed therethrough.